1. Field of the Invention
This invention relates to a method for coating a plurality of webs having discontinuous areas in the surface, e.g., arising from lengthwise connections of the webs in series with adhesive tapes or the like and which are continuously conveyed during the coating, using various kinds of coating solutions. More precisely, this invention relates to a method for applying coating solutions onto webs having discontinuities, preventing coating defects which tend to be caused downstream of the discontinuities.
2. Description of the Prior Art
Manufacture of films, papers, tapes and the like products by coating various kinds of coating solutions on webs of plastic film, paper, metal plate or the like and drying the coated layer is extensively carried out in various fields of the manufacture of photographic films, photographic papers, magnetic recording tapes, adhesive tapes, no-carbon papers and PS plates.
In the manufacture of these products, it is advantageous to substantially continuously carry out the coating operation, and therefore, the coating operation is actually successively carried out without a pause, generally, by continuously conveying webs of limited lengths which are connected lengthwise in series to a coating zone where the connected webs are coated with a coating solution. In connecting the webs, it is known that the so-called butt joint where webs are connected end to end with a splicing adhesive tape is preferred, that the splicing tape is applied more preferably on the surface of the web to be coated than on the opposite surface thereof, and that the surface of the splicing tape is preferably previously treated thereby to render the surface character of the splicing tape very similar to that of the surface of the web to be coated.
However, when a fluid coating solution is coated on the connected portion of the webs in a continuous coating operation, some seriously defective phenomena are known to often occur mainly in the area following the connected portion of the webs.
More precisely, one defect in the continuous coating of connected webs is that air is trapped between the surface of web and the coated layer in the area just following the connected portion, forming bubbles therein, and the formed bubbles adhere to coating nozzles, afterwards causing formation of streaks or the like on the surface of the coated layer extending over a fairly long distance in the surface of web. Another defect is the occurrence of uncoated or extremely thinly coated areas and a subsequent localized too thickly coated area on the surface of web in the area just following the connected portion, due to a step-like discontinuity in the surface evenness between the trailing edge of the splicing tape and the adjacent web surface and to the above-described air bubbles formed in the coated layer. The too thickly coated area requires a longer time for drying in the subsequent drying step, as compared with other areas which are coated normally. When the drying step is insufficient for drying the too thickly coated area, the thickly coated area remains undried and the undried coating solution thereon is transferred to rollers and other parts of the equipment used, thus contamination of the equipment in the subsequent steps, and ultimately other normally coated areas of the webs are stained. Thus, the too thickly coated area containing undried coating solution consequently causes a fatal defect in the quality of the product as mentioned above. Therefore, when the apparatus is stained, it is necessary to stop the manufacture procedure to clean the apparatus, which results in extreme reduction of manufacturing efficiency. On the other hand, if the occurrence of the undried area is to be prevented, the drying step must have an unneeded capacity for sufficiently drying the localized too thickly coated area. However, the thickly coated area cannot be used for manufacture of commercial products and must be discarded. Therefore, it is extremely uneconomical to spend money for drying capacity for this portion which is to be discarded.
In the above explanation, one embodiment is given where a plurality of webs is butt spliced. In addition, when the surface of web to be coated has projections or step-like discontinuities in surface level or other discontinuities, analogous coating difficulties such as coating specks, e.g., very small uncoated or too-thickly coated areas, or the like occur.
Some devices have heretofore been proposed for preventing the above described coating difficulties resulting from discontinuities in the splicing tape or the like.
One is suggested in U.S. Pat. No. 3,531,362. The outline of the method is as follows: prior to coating a solution on a web containing discontinuities (1) an oily-hydrophobic substance is applied on the discontinuous area and the adjacent web surface and the area following the discontinuous area, or (2) the splicing tape is inclined from the leading edge to the following edge to eliminate the level difference formed between the end portion of the discontinuous area and the substrate film.
In the above method (1), however, the oily-hydrophobic substance is, in general, incompatible with an aqueous coating solution, often causing adhesion difficulties of the coated layer. Also, there is the danger that the coated layer will peel off after drying due to a slight external shock in the steps following the drying step. In addition, this method requires the additional treatment of the surface of the discontinuous area with the oily-hydrophobic solution and this operation includes a danger of contamination of apparatus and other areas of the webs due to the use of the treating solution. Furthermore, a drying device having a larger capacity than necessary, as the case may be, is required for fully drying the coated pre-treating solution.
On the other hand, method (2) is not practical, in view of the shape and thickness of the splicing adhesive tape. More particularly, the splicing tape used for connecting webs generally comprises a substrate having a thickness of 10 to 50 .mu. or so coated with an adhesive. The total thickness of the tape comprising the substrate and adhesive is at most 30 to 100 .mu. or so. For attaining a sufficient effect in this method, it is necessary to incline even the adhesive portion, but it is extremely difficult to incline the trailing edge of such a thin tape even though the inclination is effected before or after the tape is adhered to the webs.
In addition, a method for varying the thickness by packing the space between the trailing edge of the tape and the surface of web with a rubber-cement material after the tape has been adhered to the web is complicated, and moreover, this method has the additional disadvantage that other areas of the webs as well as the equipment are contaminated, which is similar to the above described method where treatment with an oily-hydrophobic substance is carried out.
Another method is disclosed in British Pat. No. 1,243,663. In this method, water is applied to at least the trailing edge of a splicing tape in a previous spraying or coating, and a coating solution is coated before the water is completely dried.
However, this method has defects in that (a) a complicated apparatus is required to detect the splicing tape just before the area to be coated and to apply water just to the trailing edge of the splicing tape, (b) the layout of the passage of the web in the vicinity of the area to be coated is limited, since the surface of the web cannot be supported with a roller or the like in contact therewith after water has been applied thereto, and (c) when the web does not absorb water well, water drops on the web are transferred to the coating device such as coating nozzles, adversely affecting the subsequent coating operation.